feat(api): Ability to configure via setup() function

[mrjones2014]

Resolves #11

[glepnir]

these codes are duplicated .

 if vim.tbl_islist(cfg) then
          for _, linter_cfg in ipairs(cfg) do
            cfg_handler = cfg_handler:lint(linter_cfg)
          end
        else
          cfg_handler = cfg_handler:lint(cfg)
        end

We can only need to handle one case (table). Reduce if jump instructions. you know. The cpu can't handle the jump instruction of if correctly lol

function as_table(t)
  return vim.tbl_islist(t) and t or {t}
end

then it can be for _, linter_cfg in ipairs(as_table(cfg) do . and same as other check like fmt append can we write a function like this make these code simple ?

cfg_handler:register(key, cfg)

[glepnir]

btw thanks for pr.

[mrjones2014]

Pushed some changes -- let me know if it looks okay now!

[glepnir]

thanks I had add this support and add you in commit message close.